1. Field of the Invention
The present invention relates to an altimeter which detects the pressure of atmospheric air, and which obtains an altitude from the detected pressure value. More particularly, the present invention relates to a movement detecting type of (in other words, a stopped state detecting type of) altimeter which obtains an altitude while detecting movements in such a manner that the influence of variation in atmospheric pressure when there is no movement is minimized.
2. Description of the Prior Art
It is well known that an altitude is obtained from an atmospheric pressure detected with a pressure sensor by a standard pressure altitude conversion equation on the basis of a standard atmospheric pressure model which is formed by assuming that atmospheric air is an ideal gas, and in which a decrease by a 0.6 degree in temperature occurs with respect to every 100 m increase in altitude. It is also known that in actuality the pressure of atmospheric air is not constant and varies depending on weather, seasons, and time periods.
With respect to movement-detecting altimeters mounted on vehicles such as motor vehicles and bicycles, it is known that, by considering variations in atmospheric pressure depending on such factors, the influence of changes in atmospheric pressure is minimized in such a manner that while the altitude is obtained from atmospheric pressure by attributing a variation in the detected pressure value from a pressure sensor to a variation in altitude when the vehicle is moving, the altitude is regarded as constant even if the detected pressure value from the pressure sensor varies when the vehicle is stopped (e.g., JP 8-285582 A, JP 8-261755 A, and JP 2000-131061 A).
In this kind of movement-detecting altimeter, however, a variation in pressure in an atmosphere detected with the pressure sensor during moving is attributed to a change in altitude in practice, and, therefore, a variation in atmospheric pressure during moving is regarded as a change in altitude even when the atmospheric pressure itself varies, resulting in an error in measured altitude value. In many cases, the possibility of the influence of variation in atmospheric pressure being canceled out by averaging is high. In actuality, however, there is a possibility that errors resulting from variation in atmospheric pressure will be accumulated. For example, there is a certain degree of regularity in variation in atmospheric pressure in a day depending on the so-called sun effect. Therefore, errors can accumulate easily when there is a movement in a particular time period in a day. Similar error accumulation also occurs in a case where an atmospheric pressure change pattern and movement timing synchronize with each other.
Therefore, there is a possibility of peculiar accumulation of errors during a certain period in the movement-detecting altimeter operating as described above such that the altitude determined by the altimeter is largely different from the actual altitude. JP 2000-131061 refers to various efforts made to eliminate the influence of variation in atmospheric pressure. However, it is essentially difficult to solve similar problems as long as the difficulty in accurately detecting variation in atmospheric air in practice exists.
The present invention has been made in view of the above-described circumstances, and an object of the present invention is to provide a movement-detecting altimeter designed so as to be capable of avoiding excessive accumulation of errors.
In order to attain the above-mentioned object, a movement-detecting altimeter according to the present invention includes pressure detection means for detecting pressure of atmospheric air, reference altitude computation means for computing a reference altitude from a pressure detected by the pressure detection means, movement detection means for detecting whether or not a moving condition is maintained, and movement dependent altitude updating means for updating an indicated altitude when the moving condition is maintained, the movement dependent altitude updating means ordinarily avoiding updating of the indicated altitude when the moving condition is not maintained, the movement dependent altitude updating means having: estimated altitude computation means for obtaining an estimated altitude by regarding as a change in altitude a variation in reference altitude obtained by reference altitude computation means from the latest detected pressure value and the preceding detected pressure value obtained by the pressure detection means when the moving condition is maintained; abnormality determination means for making a determination as to whether or not the estimated altitude is an abnormal value; and altitude indication setting means for setting, as an altitude to be indicated, a designated altitude based on the reference altitude when the abnormality determination means determines that the estimated altitude is an abnormal value, and for setting the estimated altitude as an altitude to be indicated when the abnormality determination means determines that the estimated altitude is not an abnormal value.
The movement-detecting altimeter in accordance with the present invention is provided with xe2x80x9cestimated altitude computation means for obtaining an estimated altitude by regarding as a change in altitude a variation in reference altitude obtained by reference altitude computation means from the latest detected pressure value and the preceding detected pressure value obtained by the pressure detection means when the moving condition is maintainedxe2x80x9d and is therefore capable of practical real-time gasping of a change in altitude accompanying a move. The movement-detecting altimeter in accordance with the present invention is also provided, in particular, with xe2x80x9cabnormality determination means for making a determination as to whether or not the estimated altitude is an abnormal valuexe2x80x9d and xe2x80x9caltitude indication setting means for setting, as an altitude to be indicated, a designated altitude based on the reference altitude when the abnormality determination means determines that the estimated altitude is an abnormal valuexe2x80x9d. Therefore, if the accumulation of error becomes excessively large as the estimated altitude computation means repeats estimation to obtain estimated altitudes, a determination is made as to the existence of an abnormality value to check excessive accumulation of errors and to forcibly reset to the standard altitude at the corresponding time, thus grasping the altitude by removing the influence of an accumulated error. Therefore, while the advantage of the movement-detecting altimeter that the influence of variation in atmospheric pressure in a stationary condition is minimized is utilized, excessive accumulation of errors in indicated altitude peculiar to the movement-detecting altimeter (excessive accumulation of errors accompanying variation in atmospheric pressure during movement) can be avoided practically effectively.
The pressure detection means comprises a pressure sensor which is typically a piezoelectric element or a strain gage. The pressure sensor may be any transducer if the transducer is capable of directly or indirectly converting pressure into an electrical signal. Typically, the output signal from the pressure sensor is converted into a digital signal by an A/D converter by considering data processing. However, the output signal in the analog state may undergo data processing if necessary. Detection of pressure is performed at time intervals sufficiently shorter than the shortest time period in which variation in atmospheric pressure accompanying a change in altitude due to a move or variation in atmospheric pressure accompanying a change in weather, etc., can occur. Typically, detection of pressure is repeated at regular sampling intervals. If such a time interval requirement is satisfied, the pressure detection sampling (time) intervals may not be regular. The sampling intervals may be extremely short. However, for the purpose of minimizing the energy consumption such as power consumption, the sampling intervals may be set comparatively large provided that the above-described requirement is satisfied.
The reference altitude computation means obtains an altitude from pressure typically on the basis of the following (Equation 1) and (Equation 2) each time pressure detection is performed by the pressure detection means.
H=f(P)xe2x80x83xe2x80x83(Equation 1) 
f(P)=44332{1xe2x88x92(P/1013.25)0.1903}xe2x80x83xe2x80x83(Equation 2) 
where P is a detected pressure in hectopascal (hPa), and H is an altitude in meters. This (Equation 2) is an atmosphere-altitude conversion equation specified by the International Civil Aviation Organization (ICAO) on the basis of the international standard atmosphere (ISA) model in which a decrease by a 6.5 degree in temperature occurs with respect to every 1000 m increase in altitude. In this specification, the altitude obtained in this manner will be referred to as standard altitude.
In actual atmosphere, atmospheric pressure varies under the influence of weather (high atmospheric pressure, low atmospheric pressure, typhoon, etc.), etc. For example, atmospheric pressure varies in the range from 990 hPa to 1030 hPa, i.e., by about plus or minus 20 hPa (by about plus or minus 150 m in terms of standard altitude) at a height of 0 m above the sea level at which the standard altitude is about 1013 hPa. Therefore, in the present invention, an amount of error corresponding to such a range is not regarded as in excess or excessively large.
The average atmospheric pressures in different regions on the earth may differ from each other even at the same altitude. Also, even in one region, the average atmospheric pressure may vary with respect to seasons. A user may previously make a setting and a correction by considering such variation in atmospheric pressure to achieve accurate altitude indication under certain restricted conditions. In such a case, on the basis of an altitude H0 set by the user and standard altitude Hs obtained from (Equation 1) and (Equation 2) shown above, an offset xcex94Hu is determined in advance by
xcex94Hu=H0xe2x88x92Hsxe2x80x83xe2x80x83(Equation 3) 
Also,
He=Hs+xcex94Huxe2x80x83xe2x80x83(Equation 4) 
that is,
He=f(P)+xcex94Huxe2x80x83xe2x80x83(Equation 5) 
An effective standard altitude He may be obtained from this (Equation 5) and (Equation 2) shown above, i.e.,
f(P)=44332{1xe2x88x92(P/1013.25)0.1903}xe2x80x83xe2x80x83(Equation 2) 
From the above, a reference altitude Hk is defined which is formed by standard altitude Hs or effective standard altitude He.
The movement detection means may be designed so as to be suitable for detection of a moving or stopped state of a certain kind of object on which the altimeter is mounted, e.g., an automatically traveling vehicle such as a motor vehicle, a vehicle such as a bicycle, a thing worn on a person, etc. If it is used to detect a movement of vehicle, it may be a detector capable of directly or indirectly detecting revolutions of a wheel or an axle. On the other hand, to detect a movement of a walking person, a detector capable of detecting acceleration or inclinations, for example may be used. In any case, the movement detection means determines whether or not the object (a motor vehicle, a bicycle, or a walking person in the above-mentioned example) on which the altimeter is mounted is moving, and issues a movement detection signal if the object is moving. If the object is not moving, the movement detection means may output a signal indicating that the object is not moving (is stopped). Alternatively, it may simply stop transmitting the movement detection signal. Needless to say, it may issue a stoppage detection signal when the object is stopped and may stop outputting the stop detection signal when the object is not in the stopped state (is moving).
The movement dependent altitude updating means updates the indicated altitude if the result of movement detection by the movement detection means indicates that the object is in a moving state. Ordinarily, the movement dependent altitude updating means does not perform updating if the object is not moving. That is, when the object is not moving (when it in the stopped state), even if there is a variation in the pressure detected by the pressure detection means, the movement dependent altitude updating means regards this variation as due to a change in atmospheric pressure, and does not perform updating of the indicated altitude. For example, in a case where the stopped condition is maintained after initialization of the altimeter, the indicated altitude is maintained at the same value as that of the initial reference altitude regardless of the level of the detected pressure at the time of detection.
However, when the object is not moving, an altitude to be indicated may be estimated as a value which is considered to be constant, and abnormality determination processing may be performed which is similar to that performed in a case where there is a movement such as described below, while the indicated altitude value is unconditionally maintained (not updated). In a case where a considerably large amount of error is included in the indicated altitude, a determination is thereby made, for example, as to whether the difference between the indicated altitude and the reference altitude is so large as not to be ignored. If the difference exceeds a threshold value, the indicated altitude is forcibly changed to the reference altitude, for example. In such a case, xe2x80x9cavoiding updating of the indicated altitude when the moving condition is not maintainedxe2x80x9d results. xe2x80x9cOrdinarilyxe2x80x9d denotes a situation where the difference between the indicated altitude (estimated altitude) and the reference altitude does not exceed the threshold value.
On the other hand, when the object moves, indicated altitude updating processing is performed to follow a change in altitude accompanying the movement. Basically, this update processing is performed by the estimated altitude computation means under the supervision of the abnormality determination means. If an abnormality is determined, an indication setting is forcibly performed by the altitude indication setting means.
The estimated altitude computation means obtains an estimated altitude by regarding as a change in altitude a variation in reference altitude obtained from the latest detected pressure value and the preceding detected pressure value obtained by the pressure detection means when the moving condition is maintained. Typically, the xe2x80x9cprecedingxe2x80x9d detected pressure value is the immediately preceding detected pressure value. However, if variations which cannot be ignored occur due to some kind of noise in the detected pressure values obtained at different times, each detected pressure value may be replaced with a different value such as a moving average value obtained from the corresponding pressure value and a plurality of the detected pressure values before and after this pressure value. As xe2x80x9cvariationxe2x80x9d, typically xe2x80x9cdifferencexe2x80x9d is used. However, a different quantity such as xe2x80x9cdeviation from this pointxe2x80x9d may be used if necessary.
That is, when the moving condition is maintained, if the pressure at the ith sampling time ti is Pi, typically Hki=Hsi=f(Pi) with the assumption that the reference altitude Hk coincides with standard altitude Hs. Also, Hki=Hsi+xcex94Hu=f(Pi)+xcex94Hu with the assumption that the reference altitude Hk coincides with the effective standard altitude He. In either case, a change in altitude xcex94H is
xcex94H=Hkixe2x88x92Hkixe2x88x921=f(Pi)xe2x88x92f(Pixe2x88x921)xe2x80x83xe2x80x83(Equation 6) 
Accordingly, when the moving condition is maintained, an estimated altitude Ha is typically obtained by the following equation on the basis of the indicated altitude Hd determined and indicated at the immediately preceding sampling time.                                                         Ha              =                              xe2x80x83                            ⁢                                                Ha                  i                                =                                                      Hd                                          i                      -                      1                                                        +                                      Δ                    ⁢                                          xe2x80x83                                        ⁢                    H                                                                                                                          =                              xe2x80x83                            ⁢                                                Hd                                      i                    -                    1                                                  +                                  {                                                            f                      ⁡                                              (                                                  P                          i                                                )                                                              -                                          f                      ⁡                                              (                                                  P                                                      i                            -                            1                                                                          )                                                                                                                                                    (                  Equation          ⁢                      xe2x80x83                    ⁢          7                )            
The estimated altitude Ha does not depend directly on use of one of the standard altitude Hs and the effective standard altitude He as reference altitude Hk. However, the indicated altitude Hdixe2x88x921 set at the immediately preceding sampling time ordinarily depends on use of one of the standard altitude Hs and the effective standard altitude He as reference altitude Hk. Therefore the indicated altitude Hdi or Hdixe2x88x921 reflects the influence of the selection of the standard altitude Hs or the effective standard altitude He.
The above-described computation of estimated altitude Ha by the estimated altitude computation means involves the risk of accumulating errors. That is, for example, in the estimated altitude computation means, all the differences between pressures Pi and Pixe2x88x921 obtained at two consecutive sampling times while movement is continued are attributed to changes xcex94H in altitudes. Therefore, if a change occurs in atmospheric pressure accompanying a change in weather or due to the sun effect, the entire change is incorporated as an estimated altitude difference. For this reason, if the time period during which altitude estimation is performed by the estimated altitude computation means and the time period during which an increasing (or decreasing) trend of atmospheric air is exhibited coincide cumulatively with each other from start to end, the change in atmospheric pressure corresponding to the trend results directly in an error in estimated altitude. The possibility of such an accumulation or error is particularly high, for example, in a case where the frequency of moves made in a time period in a day during which an increasing (or decreasing) trend of atmospheric pressure due to the sun effect is exhibited is much higher than the frequency of moves made in another time period during which a decreasing (or increasing) trend of atmospheric pressure is exhibited, or in a case where moves are made at a high frequency when there is either an increasing trend or decreasing trend of atmospheric pressure.
The abnormality determination means detects as an abnormality an abnormally high or an abnormally low value of estimated altitude Ha due to such accumulation of error, thus making a determination as to whether or not estimated altitude Ha is an abnormal value.
Typically, the abnormality determination means compares estimated altitude Ha with reference altitude Hk at the corresponding time and checks whether the difference therebetween, the magnitude of xcex94Hak=Haxe2x88x92Hk exceeds the reference value, i.e.,. a threshold value C, thereby determining whether or not estimated altitude Ha is an abnormal value. As threshold value C, a suitable value C1 of about {f(1030)xe2x88x92f(990)}/2, for example is used. If, for example, C=xcex1C1, xcex1 is about 1 to 3, e.g., 1 or 2. Threshold value C may be varied with respect to regions, seasons, etc. The value of xcex1 may be increased with a lapse of time from a start of movement (e.g., the number of sampling times) to allow an increase in uncertainty to some extent. Abnormality determination using the abnormality determination means is made to determine whether there is a deviation in estimated altitude exceeding a deviation in estimated altitude corresponding to ordinary variation in atmospheric pressure accompanying an ordinary change in weather. In comparison of variation, a ratio (Ha/Hk) may be used in place of difference (Haxe2x88x92Hk) (needless to say, threshold value C is determined in advance with respect to the magnitude of ratio). In such a case, even if there is variation in average atmospheric pressures with respect to seasons for example, the influence of seasonal variation on the determination result is comparatively small.
The sum of standard altitude Hs and a variable offset level (or the product of these values), for example may be used as the value for comparison with estimated altitude Ha in the abnormality determination means. This variable offset level (referred to as baseline variable xcex94Hb herein) may represent, for example, an atmospheric pressure variation pattern or trend obtained on the basis of at least one of factors: an atmospheric air variation trend in a stoppage period including a point in time immediately before a start of moving (period during which there is no movement), a variable pattern having a period in a day, and seasonal variation. Data on such variations may be computed from measured values measured immediately before, e.g., from a trend immediately before, or may be data organized in a predetermined form and stored in data storage means, e.g., data depending on regions of seasons, or data on the sun effect useful in a wider range. Also, if generally real-time weather information for neighboring regions can be obtained, such atmospheric pressure variation information may be used. Although such atmospheric pressure variation information is not suitable for use in absolute-value form, variation of its relative value, a variation pattern therein or the extent of variation therein may be incorporated in the baseline variable value.
The altitude indication setting means sets, as altitude Hd to be indicated, a designated altitude based on reference altitude Hk if the abnormality determination means determines an abnormal value by recognizing excessive accumulation of errors in estimated altitude Ha. Typically, reference altitude Hk itself is designated as altitude Hd to be indicated. The possibility of this reference altitude Hk deviating excessively largely from the actual altitude is extremely low because of the definition expressed in (Equation 1) and (Equation 5) shown above. Therefore the probability thereof is high as long as a certain amount of error is accepted in practice. However, while reference altitude Hk is used as a basic reference, a baseline variable xcex94Hb such as that described above may be added to designate Hk+xcex94Hb as altitude Hd to be indicated. There is a possibility that addition of this baseline variable xcex94Hb causes a somewhat increase in error. However, baseline variable xcex94Hb itself has no effect of cumulatively increasing error. Also with respect to this case, the probability can be said to be high if a certain amount of error is accepted.
The altitude indication setting means sets an estimated altitude as an altitude to be indicated when the abnormality determination means does not make abnormal value determination. If the movement-detecting altimeter does not have the abnormality determination means, that is, does not evaluate an error, such a setting itself may be a cause of accumulation of error. The movement-detecting altimeter having the abnormality determination means is capable of avoiding excessive accumulation of errors by the abnormality determination means. As long as the estimated altitude is within a practically appropriate range, a change in pressure during movement can be taken in as a change in altitude to be included in the altitude indication value (indicated altitude).
Typically, on atmospheric pressure data detected at predetermined sampling intervals as described above, abnormality determination by the abnormality determination means is made each time the data is obtained. However, timing of abnormality determination may be positively controlled if necessary. Such abnormality determination timing control may be generally considered to be control by abnormality determination control means. This abnormality determination control means may be arranged to suspend abnormality determination processing by the above-described determination means for a predetermined time period, for example, after a time when a user sets an altitude value, instead of making the above-described determination means perform abnormality determination at certain sampling intervals. It is thought that when a user dare to set an altitude value, the user makes a setting on the basis of reliable information or makes a special setting by considering special weather conditions for example. It is also thought that atmospheric pressure does not change abruptly under ordinary conditions and therefore an offset value adjusted when a user makes atmospheric pressure setting may function effectively for a substantially long time. For this reason, the above-described arrangement may be adopted. Also, abnormality determination timing may be controlled in such a manner that after the indicated altitude has been temporarily adjusted to a reference altitude or the like according to the result of abnormality determination performed by the abnormality determination means, the subsequent intervals for performing abnormality determination may be increased, for example, abnormality determination may be performed one time in a week or in a day.
The arrangement may be such that when an abnormality value is determined by abnormality determination means, the altitude indicating setting means does not initialize or reset the indicated altitude but alarm means notifies a user of the determination result. In this case, the movement dependent altitude updating means has xe2x80x9cestimated altitude computation means for obtaining an estimated altitude by regarding as a change in altitude a variation in reference altitude obtained from the latest detected pressure value and the preceding detected pressure value obtained by the pressure detection means when the moving condition is maintained; abnormality determination means for making a determination as to whether the estimated altitude is an abnormal value; and alarm means for notifying a user that an abnormality determination is made when the abnormality determination means determines an abnormal valuexe2x80x9d. An operation mode designation means such as a pushbutton switch may be provided to enable selective designation of a state in which the movement dependent altitude updating means includes the altitude indication setting means, and a state in which the movement dependent altitude updating means includes the alarm means. The arrangement may alternatively be such that the movement dependent altitude updating means includes both the alarm means and the altitude indication setting means, and that while alarming is performed, the indicated altitude setting processing is continued by the altitude indication setting means.
The movement-detecting altimeter may be a type mounted on a vehicle or a type carried by a person, or may be provided in other forms.